There are really just two kinds of relationships: One where you trust someone, and one where you don’t. Of course, just because there’s trust doesn’t mean the relationship will work out, but if there isn’t, it’s almost guaranteed to fail.

The trouble is, what does it even mean to trust your partner nowadays, given the rapidly shifting makeup of relationships, the prevalence of mixed-gender friendships, and the increasing pressure to be progressive and relaxed about commitment? Such a question arises in an essay at Slate, where Ben Mathis-Lilley asks, “When is it OK to be alone with another person’s spouse?”

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The author posed this question to his own friends to gauge whether this was a common fear, and while most of them agreed an hour alone with the cable guy was hardly a threat to their relationship, they admitted there were similar enough situations that could be. Mathis-Lilley writes:

Maybe it wouldn’t bother you if your spouse were home with a repair person—but what if he/she mentioned stopping by a single colleague’s house to finish up a project they were working on together? What if your life partner and an acquaintance, sports fans both, watched the Big Game by themselves? What if an attractive area parent brought his or her kid over for a play date while you were at work? What if you were out of town for the weekend and your spouse and a single friend went to see the buzzy movie du jour?

As for the responses, some of the men and women he polled said the situations didn’t seem “quite right,” and while some said they’d claim to be okay with it, they confessed they really wouldn’t.

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He asserts that these friends are not particularly conservative people, nor did they actually believe that their spouses would be up to no good in these hypotheticals. Not all were bothered by any theoretical jealousy-inducing scenario, but those who were said the reasons behind it were pretty simple: jealousy not only of the third party, but of the spouse getting to have fun without them—particularly for people raising families. Others said it would be fine so long as the person in question weren’t too attractive.

But in all, this really points to a conundrum today’s couples face that Mathis-Lilley identifies as uniquely modern:

What might be happening is that our jealous reptile brains haven’t caught up with new social rules and gender roles. These hypotheticals are situations—men and women traveling together for business; a married man being a primary caregiver for children during the day; women identifying as sports fans—that to greater and lesser degrees didn’t exist until relatively recently. As historian Stephanie Coontz of Evergreen State College noted when I asked her about the subject, the Industrial Revolution–era doctrine of separate men’s and women’s spheres limited interaction between the sexes for a large chunk of American history: Men’s spheres were public life and the workplace, and women’s was the home. Marriages were often arranged through families and preceded by formal “courtship.”

Career and dating mores have obviously changed considerably, and “there are more opportunities for male-female socialization in the United States now than ever before.”

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You don’t have to be married to have stared down this particular relationship issue: who hasn’t been jealous of the friend of their significant other, either the one they used to date, or almost dated but never quite did, or who just seems to have always harbored some kind of crush for your partner? And we can’t have this conversation without mentioning Millennials, who have been credited with unbundling relationships to the point that every single aspect of coupling can be gotten a la carte: one can have multiple people for chill hangs, casual sex, a long-term partnership, even child-rearing.

Regardless, none of these scenarios actually change what it means to trust someone, which involves a mix of two things, really: trusting someone, and behaving in a trustworthy manner. If that seems pretty 101, it’s because it is. Yet it’s remarkable how often people expect blind trust in relationships either right from the start, or when they themselves don’t behave in a trustworthy way. Trust is built over time, according to the leading researcher on the subject, and one if its major components is the ability to really get how your actions affect someone else. It’s also remarkable how little someone can trust their partner even when they do behave largely consistently (yes, everyone’s been burned, but you should still trust people.)

I’m a firm believer in negotiated ideas of trust, because everyone defines it differently. I knew a girl who thought just kissing someone wasn’t cheating; I know a guy who has Friday night “friend dates” with women he meets at work even when he has a girlfriend, even when he thinks they are cute. His past girlfriends, he claims, have all been fine with it. I can think of a lot of people who wouldn’t be.

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So trust and negotiation can pave this road pretty well, but let’s say you’re trustworthy and trusted, and the same goes for your partner, and you embrace the idea of a long leash—letting someone be in the world with autonomy, because otherwise you’re suffocating them—and then, suddenly, they are hanging out alone with the opposite or same sex, depending. You want to be relaxed, but your brain is going nuclear—how do you quell the irrational fear knowing full well that you can’t be territorial?

The only thing you can do is rely on the trust you’ve built, and hope it’s holding up. Assume that, as you’ve negotiated, the person is going to not do whatever you’ve agreed is not okay, and that they will be open enough to tell you if they are tempted. And even if they don’t behave exactly as you’d like, that you can still trust them.

A lot of it, I think, depends on how comfortable you are with whether people in such scenarios can actually be friends. I think they can. I think it’s true far more often than not. If you don’t, you will likely never be totally at ease with such an arrangement. Back at Slate, Mathis-Lilley concludes that historically speaking, there’s never been a clear-cut standard on the impropriety of spending time alone with the desired other sex of your choosing.

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“It’s up to us, as always, to keep each other happy by working out the rules on our own,” he writes. And he’s right: Talking it out tends to be the best way through these thorny issues. That might be the least sexy answer, but it’s also the only one that has ever seemed to work.

Illustration by Tara Jacoby.